The present invention relates to an interface circuit between a four wire line and a two wire line, including a four wire interface with a receive input and a send output, as well as a two wire interface including two terminals facing the two wire line, one for each wire. An output amplifier is located before each terminal. First circuit means apply, via the amplifiers, a signal received on the receive input as a differential signal on said terminals of the two wire interface. Second circuit means feed to the send output a differential signal received on said terminals of the two wire interface from the two wire line. Third circuit means form a longitudinal signal by means of the sum of voltages appearing on each of said terminals. A feed back loop extends to each amplifier for an adjustment signal which is derived from the longitudinal signal, for adjusting the impedances of the two wire interface to earth, the so called longitudinal terminating impedances so as to attain longitudinal balance.
Generally, a subscriber line is connected to a telephone exchange office via a so called two wire interface. The telephone exchange office is connected to other telephone exchange offices via a so called four wire interface. Two of the wires are used for sending speech signals to other exchange offices and the other two for receiving speech signals from other exchange offices.
A speech signal which is received on the four wire interface is passed on to the two wire interface where it appears as a transversal (differential) signal. If a transversal signal arriving from the subscriber appears in the two wire interface it will be passed on to the four wire interface and possibly further on to another exchange.
Today, integrated circuits are used for performing the transformation between the two and four wire interfaces. Such a circuit usually is denominated SLIC, Subscriber Line Interface Circuit. There are very high demands put on the SLIC that its so called longitudinal terminating impedances should be equal, i.e. the impedance from the two wire interface to earth should be equal for the wires of the two wire line.
External disturbances, e.g. from the power network or caused by a thunderstorm, may give rise to so called longitudinal signals on the two wire line. Besides that these longitudinal signals may disturb the speech signals, they may also give rise to harmful voltages in the two wire interface. Inversely, transversal signals, e.g. speech signals, may give rise to longitudinal signals. This may cause cross talk to two wire lines located nearby.
In order to obtain longitudinal balance good enough in a SLIC, trimming of amplifications in a number of signal paths has been used, cf. the Swedish patent 448,264, which discloses a circuit essentially of the kind defined by way of introduction. The trimming may be performed in several ways during the manufacture, e.g. by trimming thin film resistances by means of a laser.
A circuit likewise of essentially the kind defined by way of introduction is disclosed in the European patent 0,134,229, in which a method is described for automatically trimming the circuit in its location in the exchange office. More particularly, this method requires that the circuit is put into a trimming state, whereupon a number of signal connecting up and measuring operations followed by trimming adjustments are performed.
As an example of the general state of the art the following patent publications may besides be mentioned: SE-B 446,579, EP-A2 272,800, U.S. Pat. No. 4,387,273 and WO-A1 90/01837.